1. Field of the Invention
The present invention generally relates to semiconductor chip manufacture, and more particularly to methods of making stacks of chips by lamination of wafers.
2. Background Description
Since the development of integrated circuit technology, computers and computer storage devices have been made from wafers of semiconductor material comprising a plurality of integrated circuits. After a wafer is made, the circuits are typically separated from each other by dicing the wafer into small chips, each containing one or more circuits. Thereafter, the individual chips are bonded to carriers of various types, and interconnected by wires to form modules. Such "two-dimensional" modules of chips fail to optimize the number of circuits that may be packaged in a given space and introduce undesirable signal delays, capacitance, and inductance as signals travel between chips.
Recently, three-dimensional arrays of chips have emerged as an important packaging approach. A typical multichip electronic module comprises multiple integrated circuit (IC) chips adhesively secured together as a monolithic structure. A metallization pattern is often provided directly on one (or more) side surface(s) of the module for IC chip interconnections and for electrical connection of IC chips to circuitry external to the module. The metallization pattern can include both individual contacts and bussed contacts. Multichip modules comprising stacks of IC chips are referred to herein as "stacks".
The current stack fabrication process suffers from problems that negatively effect overall stack manufacturing yield and efficiency. As a result, stack manufacturing costs remain high and profit margins remain low. Several of these problems involve: (1) IC chips becoming unusable in the stacks due to edge chipping during wafer dicing; (2) difficult side-surface channel via fabrication; (3) side-surface polyimide edge bead thickness; and (4) variable T-connect quality.
In addition, the current stack fabrication process requires much tighter dicing tolerance than is currently required for IC chip dicing associated with single IC chip, plastic encapsulation type packaging. The dicing tolerance for plastic packaging is approximately, for example, .+-.20 .mu.m, while the stack process requires a dicing tolerance of approximately, for example, .+-.5 .mu.m.
When IC chips of inadequate tolerance are stacked and laminated, the varying IC chip sizes result in IC chips shifting within the stack. Such shifting causes misalignment of, for example, the transfer metal leads of the IC chip in the stack. Accordingly, the side-surface of the stack requires polishing to expose all of the transfer metal leads, thereby reducing the total number of times that the stack side-surface can be reworked. Furthermore, the misalignment of the IC chips results in wider side-surface wiring being required to "capture" all the side-surface connections. Side-surface wiring density is therefore reduced.
As yet another problem, the varying IC chip sizes force the stack/lamination fixture used for assembly of the stack to be large enough to accommodate IC chips of varying sizes up to the maximum specification limit. This increases the likelihood of IC chip shifting. Moreover, the forces on the stack during lamination are concentrated on the largest chips in the stack because they are in direct contact with the sides of the lamination fixture. Therefore, these large chips tend to become damaged during lamination. This further compromises stack yield and requires increased side-surface polishing.